4.19-stable review patch. If anyone has any objections, please let me know.
------------------
From: Salvatore Dipietro dipiets@amazon.com
[ Upstream commit 7267e8dcad6b2f9fce05a6a06335d7040acbc2b6 ]
On CPUs with weak memory models, reads and updates performed by tcp_push to the sk variables can get reordered leaving the socket throttled when it should not. The tasklet running tcp_wfree() may also not observe the memory updates in time and will skip flushing any packets throttled by tcp_push(), delaying the sending. This can pathologically cause 40ms extra latency due to bad interactions with delayed acks.
Adding a memory barrier in tcp_push removes the bug, similarly to the previous commit bf06200e732d ("tcp: tsq: fix nonagle handling"). smp_mb__after_atomic() is used to not incur in unnecessary overhead on x86 since not affected.
Patch has been tested using an AWS c7g.2xlarge instance with Ubuntu 22.04 and Apache Tomcat 9.0.83 running the basic servlet below:
import java.io.IOException; import java.io.OutputStreamWriter; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;
public class HelloWorldServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=utf-8"); OutputStreamWriter osw = new OutputStreamWriter(response.getOutputStream(),"UTF-8"); String s = "a".repeat(3096); osw.write(s,0,s.length()); osw.flush(); } }
Load was applied using wrk2 (https://github.com/kinvolk/wrk2) from an AWS c6i.8xlarge instance. Before the patch an additional 40ms latency from P99.99+ values is observed while, with the patch, the extra latency disappears.
No patch and tcp_autocorking=1 ./wrk -t32 -c128 -d40s --latency -R10000 http://172.31.60.173:8080/hello/hello ... 50.000% 0.91ms 75.000% 1.13ms 90.000% 1.46ms 99.000% 1.74ms 99.900% 1.89ms 99.990% 41.95ms <<< 40+ ms extra latency 99.999% 48.32ms 100.000% 48.96ms
With patch and tcp_autocorking=1 ./wrk -t32 -c128 -d40s --latency -R10000 http://172.31.60.173:8080/hello/hello ... 50.000% 0.90ms 75.000% 1.13ms 90.000% 1.45ms 99.000% 1.72ms 99.900% 1.83ms 99.990% 2.11ms <<< no 40+ ms extra latency 99.999% 2.53ms 100.000% 2.62ms
Patch has been also tested on x86 (m7i.2xlarge instance) which it is not affected by this issue and the patch doesn't introduce any additional delay.
Fixes: 7aa5470c2c09 ("tcp: tsq: move tsq_flags close to sk_wmem_alloc") Signed-off-by: Salvatore Dipietro dipiets@amazon.com Reviewed-by: Eric Dumazet edumazet@google.com Link: https://lore.kernel.org/r/20240119190133.43698-1-dipiets@amazon.com Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 00648a478c6a..712186336997 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -733,6 +733,7 @@ static void tcp_push(struct sock *sk, int flags, int mss_now, if (!test_bit(TSQ_THROTTLED, &sk->sk_tsq_flags)) { NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPAUTOCORKING); set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags); + smp_mb__after_atomic(); } /* It is possible TX completion already happened * before we set TSQ_THROTTLED.